Add Windows support to lit.cfg

* Enable running Flang tests on Windows

Author: kaadam

test/lit.cfg

@@ -27,10 +27,6 @@ import lit.llvm
 lit.llvm.initialize(lit_config, config)
 lit.llvm.llvm_config.with_system_environment("COMPILER_PATH")
 
-if platform.system() == 'Windows':
-    lit_config.note('We do not support Windows, but hey, congratulations on porting to Windows!')
-    raise SystemExit
-
 # Configuration file for the 'lit' test runner.
 
 # name: The name of this test suite.
@@ -45,6 +41,8 @@ if use_lit_shell:
 else:
     # Otherwise we default to internal on Windows and external elsewhere, as
     # bash on Windows is usually very slow.
+    # NOTE: Flang tests have been tested only on Windows with MS Visual Studio environment,
+    # the behaviour under Cygwin or MinGW is unknown.
     execute_external = (not sys.platform in ['win32'])
 
 # testFormat: The test format to use to interpret tests.
@@ -219,7 +217,7 @@ def getClangBuiltinIncludeDir(clang):
     dir = cmd.stdout.read().strip()
     if sys.platform in ['win32'] and execute_external:
         # Don't pass dosish path separator to msys bash.exe.
-        dir = dir.replace('\\', '/')
+        dir = dir.replace(b'\\', b'/')
     # Ensure the result is an ascii string, across Python2.5+ - Python3.
     return str(dir.decode('ascii'))